Residual stresses in welded joints of work hardening steels with high manganese content

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For the development of high strength structural components in body construction the application of steels with an optimal combination between ductility and strength is of particular interest. Steels with high manganese contents are highly interesting as the final mechanical properties are achieved through deformation induced martensite formation (TRIP effect) or through formation of twin boundaries (TWIP effect). In the deformed state these steels combine high strength with high plastic deformation reserves. Important for the design of weld constructions from instance in the field of automotive components made from these materials is that the welded joints must exhibit very high strength even for fatigue load conditions. In this regard it remains unclear to what extend welding induced residual stresses will affect the strength and fatigue strength behavior and to what extend the micro residual stresses of 2nd and 3rd kind contribute. Furthermore, it is unknown if there is a relationship between the various causes for the development of micro residual stresses and the resulting stability of macro residual stresses. By answering these questions the research project will make a significant contribution to gain a better understanding of the processing characteristics and the resulting strength of strongly work hardening austenitic steels and provides an improved basis for the quantitative consideration of different residual stress shares for components design. The project aims at the understanding and description of the interrelations between the macro residual stresses induced by the forming process and the state after welding. This will be realized by systematic experiments and through finite element simulations of the process chain deformation-welding.In this regard, extensive systematic X-ray diffraction analyses for welded samples made of steel with high manganese content, i.e. X40MnCrVAl19 2.5 (TWIP) and X10Mn7 (TRIP 700), for analyzing local phase contents, phase-specific texture evolutions and the development and stability of phase-specific residual stresses at all scales are planned. The comprehension of the relationship between process parameters and resulting micro and macro residual stresses will provide the basis for optimizing quasi-static and cyclic strength of components made from these alloys through optimizing the heat input and the cooling course and hence of the welding process itself. For the FE simulations the correlation with micro residual stresses will be provided through local calculation of plastic deformations during forming and welding and coupling with experimental results.